1. Field of the Invention
This invention relates to a method for preparing a magnetic power material or a magnetic composition containing a specific resin excellent in heat resistance, chemical resistance, etc., and a magnetic powder and a method of preparing a resin-bonded type magnet prepared by molding said magnetic powder material or magnetic composition.
The present invention also relates to a process for preparation of a resin composition for powder molding and a process for production of a powder molded product. More particularly it relates to a resin composition as a material for production of a powder molded product having heat dimensional stability, chemical resistance, and so forth nearly equal to those of a metal, and a process for producing the powder molded product with high productivity and efficiency.
2. Description of the Related Arts
Heretofore, resin-bonded type magnets have mainly been prepared by the compression molding method or the injection molding method. Among these, in the compression molding method, a thermosetting resin such as an epoxy resin, etc., has until now been used as a resin binder. However, since the thermosetting resin is extremely low in chemical stability with a lapse of time, the resulting resin-bonded type magnetic powder material is poor in preservability and also poor in production stability. Further, at preparation, it takes one hour or more for thermosetting processing so that productivity is low. Moreover, there are various problems in that a critical temperature for use of the resulting resin-bonded type magnet is 120.degree. C. or so, which is impractical, and it also lacks dimensional stability with a lapse of time.
Conventional resin-bonded type permanent magnets have mainly been prepared by the injection molding method or the compression molding method as disclosed in, for example, Japanese Patent Kokai No. 40003/1982, No. 173804/1983, No. 16002/1983, etc., and as a binder resin, a polyamide, ethylene-vinyl acetate copolymer, etc. in the injection molding method and an epoxy resin in the compression molding method have been used.
However, a resin-bonded permanent magnet using a polyamide or an epoxy resin as the binder has poor humidity resistance whereby there are problems of rust generation and diminished characteristics due to a change in dimension with a lapse of time, and its critical temperature for use is as low as 120.degree. to 150.degree. C.
As the method for obtaining a resin-bonded type magnet by cold compression molding after coating magnetic powder with a thermoplastic resin, various ones have been known. For example, in Japanese Patent Kokai No. 104254/1975, a method is disclosed in which magnetic powder particles coated with Nylon precipitated from about 2% by weight of a solution are subjected to compression molding to obtain a resin-bonded type magnet. Further, in Japanese Patent Kokai No. 186908/1983, there is disclosed a method in which ferrite series magnetic powder is suspended in a radical polymerizable monomer solution to carry out polymerization, and then the magnetic powder particles coated their surfaces with resin are subjected to compression molding in a magnetic field to obtain a resin-bonded type magnet having a magnetic anisotropy. However, these are unsatisfactory in resin characteristics such as heat resistance, chemical resistance, etc., so they could not be used practically.
In recent years, a crystalline resin excellent in heat resistance, chemical resistance and dimensional stability, such as polyphenylenesulfide (PPS), polyether ether ketone (PEEK), etc. has been known. in Japanese Patent Kokai No. 113403/1985, there is disclosed the method in which the above PPS, PEEK and magnetic powder are fused and kneaded, and then a rare earth resin-bonded type magnet is obtained by injection molding.
However, the crystalline resin such as PPS or PEEK requires a high temperature for fusion molding such as 350.degree. C. or higher, so that there is a disadvantage in that magnetic powder of the rare earth is likely to be oxidized at molding. In addition to the above, various methods have been proposed and for example, in Japanese Patent Kokai No. 134517/1974 and No. 103309/1984, there are proposed the methods in which a mixture or a coated material of thermoplastic resin powder such as a polyamide, a polyolefin, a polycarbonate, etc. and magnetic powder is subjected to hot press molding. However, in this method, when removing a molded material from the mold, it takes a long time for cooling and solidification so that there is a problem of low productivity. Also, in Japanese Patent Kokai No. 186908/1983, there is described a radical polymerizable monomer such as methyl acrylate, etc. is brought into contact with ferrite series magnetic powder to carry out polymerization, and after coating the powder surface with the polymer, cold press molding is carried out. However, since heat resistance and chemical resistance of the binder resin are insufficient, it has not yet been used practically.
Further, in Japanese Patent Kokai No. 279106/1986, a technique is proposed in which by using PPS as a binder, after PPS is fused under heating and kneaded with magnetic powder, injection molding or extrusion molding is carried out. However, in order to carry out such a fusion under heating and kneading, a treatment at high temperature for a long time should be carried out since fluidity in the kneaded material is required. This treatment results in a serious problem that the magnetic characteristics are lowered since oxidation of the magnetic powder is promoted.
In powder metallurgy, a sintered molding has heretofore been produced by a process comprising the steps of compression molding using an organic binder, degreasing, and sintering. This process, however, has disadvantages in that a large amount of energy and a long time are needed at both the degreasing and sintering steps, dimensional accuracy is poor because of its large shrinkage after sintering, and breaking and cracking readily occur.
Moreover those parts which are long, thin or grooved are difficult to produce because of dimensional changes due to shrinkage at the time of sintering.
On the other hand, copper base alloys containing zinc, such as brass and nickel silver, have a disadvantage of shortening the service life of a sintering furnace.
In order to overcome the above problems, Japanese patent Application Laid-Open No. 38641/1976 discloses a molding obtained by a powder metallurgical technique using a thermosetting resin (e.g. epoxy resin) as a binder. An epoxy resin, however, has disadvantages in that a long time is required for calcination after compression molding, the resulting molding is large in dimensional changes under an atmosphere of high temperature or high humidity as compared with a sintered metal or diecast, which is unsuitable for practical use, and the mechanical strength of the molding is small.
In order to overcome the above problems, Japanese Patent Application Laid-Open No. 104254/1975 discloses a powder metallurgical technique in which various fine powders are coated with Nylon resin (5 to 35 vol %) and then compression molded to produce a molding, so that the molding can be used as it is without degreasing and sintering. This technique, however, has not been put to practical use because the Nylon resin is poor in heat resistance, mechanical strength and chemical resistance.
For molding, therefore, the powder metallurgical technique has not been applied, and rather injection molding in which a resin content of at least 35 vol % is required has been employed in view of advantages of strength and moldability. For example, composition having a high specific gravity as prepared by charging a metal or metal oxide powder in a high concentration are disclosed in Japanese Patent Application Laid-Open Nos. 258952/1988, 202653/1988, 183956/1988 and 142051/1988. This composition, when charged with a high specific gravity metal such as iron powder or copper powder, is used in production of fly wheel or die cast moldings, and the resulting molding has a mechanical strength comparable with that of a resin and is sufficiently durable for practical use.
In the case of injection molding, suitable fluidity is needed at the time of molding and, therefore, the amount of the powder charged is limited and a high specific gravity composition cannot be attained, by charging the powder in a higher concentration. Even if the high specific gravity is attained, the resulting molding still has a larger coefficient of linear expansion than the metal or die case molding and is increased in the dimensional changes under high temperature conditions, unsuitably for practical use.
Although the injection molding is effectively employed in production of moldings having a complicated shape, in the case of production of a small number of moldings which are simplified in the shape but in various types of shapes, it suffers, from disadvantages in that abrasion of the die due to the charged powder is seriously large, leading to an increase of cost for the maintenance of the die, and no mass production effect can be obtained.
The present inventors made an extensive investigation to overcome the above problems of the prior art and to produce a powder molded product having heat stability and chemical resistance comparable with those of metals, and having a sufficiently high mechanical strength for practical use, with high efficiency.